1. Field of the Invention
The present invention relates to an optical device and, more particularly, to a laser. The present invention also relates to an optical transmitter using the optical device and a network using the optical transmitter. Furthermore, the present invention relates to a method of driving the optical device.
2. Related Background Art
As the optical communication techniques have evolved to realize larger capacities, many problems have been solved. However, so-called chirping, i.e., a phenomenon in which refractive index drifts caused by a nonuniform carrier distribution in a semiconductor laser upon high-speed intensity modulation distort the oscillation waveform, remains unsolved. In a popular method currently used for avoiding this problem, a semiconductor laser is driven continuously by coupled waves (CW), and intensity modulation is made by an external modulator consisting of a dielectric, semiconductor, and the like. With this method, a size reduction and a cost reduction of the device are limited, and when such device is used in an optical network, the flexibility (e.g., the capability of transferring signals having considerably different modulation speeds at the same time) of the optical network is not so high.
On the other hand, as another method, a method of switching the plane of polarization of oscillation light of a device in correspondence with a signal, i.e., a so-called polarized wave modulation method, is available. This method is disclosed in, e.g., Japanese Patent Laid-Open Application Nos. 62-42593 and 62-144426. The essence of this method is as follows. As shown in FIG. 12, a semiconductor laser with characteristics in which an input TM mode is converted into an output TE mode at a given current value is used. A specific current value at which TE and TM modes oscillate simultaneously is defined as a bias point, the threshold value gain of the TE and TM modes is switched by a signal current (modulated current) I.sub.S, and only light polarized in a specific direction is output onto a transmission path using a polarizer.
The polarized wave modulation method has the following essential advantage. That is, since the optical density of a semiconductor laser is constant even during modulation (since the driving current is always nearly constant and is not turned on/off), carrier drifts caused by modulation can be minimized.
However, the above-mentioned prior arts do not describe any detailed structures of such laser.
On the other hand, as an example of the arrangement, the following two-electrode arrangement is available.
(1) A conventional DFB laser is taken to have a two-electrode arrangement, and carriers are nonuniformly injected to simultaneously control the phase and gain.
(2) The active layer adopts a quantum well structure to have polarized wave dependence of the gain, thus compensating for shortage of the gain of TM modes.
(3) The TE and TM modes at the Bragg wavelength are switched by phase control to attain direct polarized wave modulation.
A laser that can switch the polarized wave of output light is also disclosed in Japanese Patent Application Laid-Open No. 2-159781.